Stretchable OLEDs for Displays, Lighting

Stretchable, bendable, foldable, organic light emitting diode (OLED) panels and displays are possible, according to University of California at Los Angeles (UCLA) researchers who recently demonstrated a transparent, elastic OLED prototype. By engineering its material stack exclusively from materials that are immune to deformation, the researchers claim future stretchable and bendable OLEDs could redefine the user experience with a new class of smartphone, smart electronic clothing, and wallpaper-like lighting panels.

Purple-gloved researcher demonstrates how stretchable OLED display can be bent, stretched, and deformed while working undamaged. (Source: UCLA)

"Imagine a smartphone that is easy fit into one's pocket, but the screen could be expanded for viewing comfort," said UCLA professor of materials science Qibing Pei in an interview with EE Times. "Being stretchable and highly flexible means that OLED lighting panels and displays could be like wallpaper -- very thin, lightweight, and collapsible into a small volume when not in use."

To prove the concept, Pei and Jiajie Liang, a postdoctoral scholar in Pei's Soft Materials Research Laboratory at UCLA, fabricated two small stretchable OLEDs -- one a lighting panel and the other a passive five-by-five pixel display. Both were demonstrated to be bendable, foldable, and stretchable while operating, surviving over 1,000 trials of stretching 30 percent larger than their original shape and size. The lighting panel and display were also demonstrated to be stretchable and bendable in any direction, surviving stretches of up to twice their original size and 180 degree bends.

The OLED lighting panel was demonstrated to be stretchable to more than twice its original size. (Source: UCLA)

To achieve a stretchable, bendable OLED, the researchers had to reengineer all the materials in an OLED stack. Pei cited two key elements that led to their success:

"First was the development of a transparent compliant conductor comprising a percolation network of silver nanowires in the surface layer of an elastomer film," said Pei. "The second key development is an OLED architecture that we call light-emitting electrochemical cell [LEC]. The LEC devices use a thin luminescent layer that is actually a mixture of electronic semiconductor and an ionic conductor."

Using low-temperature solution-based processing steps, the light-emitting electrochemical cell was sandwiched between the transparent elastic silver-nanowire composite electrodes made from a rubber-like polymer. For the display prototype, the composite electrode layers were patterned into rows on the top layer and columns on the bottom layer, thus enabling each pixel to be separately addressed.

All layers in the stretchable OLED display stack use materials that can be deformed without damage. (Source: UCLA)

According to Pei, the benefits of a light-emitting electrochemical cell over a conventional OLED is that its emissive layer can be relatively thick, thus enabling packaging techniques such as lamination during OLED fabrication. Secondly, the injection of electrons and holes is independent of the electrode material's work-function, thus their silver nanowire architecture could be employed for both hole injection at the anode as well as electron injection at the cathode.

Next the researchers are working on encapsulation methods that protect the organic materials from deterioration in the presence of air, as well as on flexible thin-film transistors (TFTs) so that active matrix displays can be fabricated using the same basic stretchable, bendable material stack.

Also contributing to the work was UCLA postdoctoral scholar Lu Li, former UCLA postdoctoral scholar Zhibin Yu (now a postdoctoral scholar at the University of California at Berkeley), and UCLA doctoral candidate Xiaofan Niu. Funding was provided by the National Science Foundation (NSF) and the Air Force Office of Scientific Research.

Thanks for including the Samsung link--I've been meaning to include them in the discussion--and you're right on both counts: their demo was very impressive, but of course the encapsulation technology will be the critical factor in how soon they can come to market.

Pei said it in the article: "The second key development is an OLED architecture that we call light-emitting electrochemical cell [LEC]. The LEC devices use a thin luminescent layer that is actually a mixture of electronic semiconductor and an ionic conductor." These devices are not like traditional colour display pixels; they don't have addressability and they don't have multi-colours (i.e. RGB) so cannot display any reasonable gamut of colour range. So these are essentially lighting panels, and the particular ones shown here are yellow. The light emission comes from the polymer, phenyl substituted poly(para-phenylene vinylene) known as "Super Yellow," (you can purchase it and make your own). Even though they show "pixels", note they are not able to light them individually, and a screen made of these would really resemble something very old! -> https://en.wikipedia.org/wiki/Monochrome_monitor

@docdivakar I would be curious to know if there are any optical behavioral shifts with stretch ratios? And does it vary with time?

I believe most of the stretching durability tests were performed on the light panel rather than the display prototype, so no data on optical behavior shifts or time variance properties yet. Also the researchers next concentration is finding an encapsulation method that can stretch as easily as the LEC.

It seems like the technology shows good promise. There are many open questions for this before product designs can start. I would be curious to know if there are any optical behavioral shifts with stretch ratios? And does it vary with time?

We are witnessing another type of display technology take shape -light-emitting electrochemical cell (LEC)...

chanj, i agree that some technologies are faster to integrate in consumer market but other such as 3D LED have failed despite all the hype and maturity. Sometime the price at which the technology is sold is not enough and technology support and peripherial support is more than necessary.

I can see a lot of applicable area and I do agree that the talk of application may be premature until the technology is commerically viable. Nonetheless, who knows how soon the technology can be commerically viable? I have noticed lately that technology is widely available sooner than before. Just look at OLED itself, the pace is far exceeded that of LCD. The market welcomes small OLED display on mobile device, then large TV.

I think that until the technology - whatever that maybe - for curved and flexible displays becomes commercially viable the the discussion of which applications will benefit is somewhat premature. Sure, flexible display technology will come at some point in the future...consider that the first LCDs were prototyped 40-50 years ago and perhaps only now have they become ubiquitous.